1. Field of the Invention
The invention relates to the field of printing systems, and in particular, to methods and systems for calibration of multiple fixed printheads in a printing system.
2. Statement of the Problem
In high speed production printing system using inkjet technologies it is common to utilize multiple printheads for various reasons. In some cases multiple printheads provide improved speed and/or improved print quality Inkjet printheads, electrophotographic toner printheads, wax printheads, etc. all have some inherent technological limits for the volume of colorant dispersed over time. Thus multiple printheads may be employed to generate a given image print quality more quickly. Or, multiple printheads may be employed to apply a larger volume of colorant over a given period of time to improve print quality (i.e., image optical density).
To meet the demand for producing the high quality images, printers are designed with a number of printheads in a fixed array arrangement, where only the media moves. This is very commonly used in high speed production inkjet printers. However, due to the technology limitations, design of faster firing printheads is difficult to accomplish. An alternate way of producing higher color image quality is to increase the number of printheads and print the same image multiple times on the media at same location. This mode of printing is usually referred as “multiple pass” printing (or simply “multi-pass”). Normal printing using a single printhead is referred to as “single pass” printing. For example, adding twice the number of printheads will enable printing the same image twice at double the speed of single pass, producing the same print quality as single pass printing.
One such dual-pass printing design uses two channels to print the same data twice on fast moving media. These two passes use different screens/halftones to print. This design also allows for controllably switching between single pass printing mode and dual-pass printing mode. The screens/halftones for single pass printing are usually calibrated to produce distinct 256 output “gray” levels on the media. However, when the two single passes print the same image twice with those calibrated screens/halftones the resulting output will not produce the desired distinct 256 gray scale levels. The shadow tone region gray scale levels may be saturated with ink (e.g., colorant) due to high dot gain. At the point in the tone range where saturation occurs, the solid area density ceases to increase. Some calibration mechanism is required to produce 256 distinct gray scale levels when printing using dual-pass.
As presently practiced, a variety or multiple pass printer calibration procedures are known. Most of the presently known calibration techniques are restricted to scanning/moving print heads. Other presently known calibration techniques discuss procedures to split and distribute the dots in screens between each passes.
None of the presently known calibrating techniques adequately address two independent fixed printheads, each of which print the entire page image, so as to assure appropriate optical density at each of the possible gray scale levels for a pixel's intensity. Thus it is an ongoing challenge to properly calibrate multiple fixed printheads operating in a dual-pass (i.e., multi-pass) mode.